Group A streptococci are still a major cause of illness in children. The resurgence of rheumatic fever and associated heart disease in many communities in the United States in the past two years has reminded the medical community that this pathogen has the potential to initiate serious morbidity. Pathogenic streptococci are covered with a fibrous protein coat which permits them to avoid the human immunological defenses. Like most bacterial pathogens the macromolecular composition of this fibrous layer and the capacity to cause disease is finely regulated and subject to change. Streptococci switch between virulent and avirulent states, a phenomenon termed phase variation. Understanding the molecular basis of phase variation and its role in the infectious process is the ultimate goal of this project. Virulence is dependent on a multitude of factors, antiphagocytic M proteins, and inhibitor of chemotaxis, IgG Fc receptor proteins and a variety of toxins. Our studies showed that expression of M protein is under the control of a reversible genetic switch and that genes which encode these proteins are clustered on the streptococcal chromosome. We propose that these genes compose a complex regulatory circuit which is controlled by a master gene, the product of the virR locus. We also suggest that this regulatory pathway coordinates the "on off" expression of these three virulence factors. To test this model the nucleotide sequence of this gene cluster will be completed. The virR gene and its role in coordinating expression and phase variation of these surface protein genes will be investigated. The impact of virR on promoter recognition and transcription of each gene will be examined by quantitating and sizing mRNA transcripts from each gene. Our studies will make use of lacZ gene fusions and insertion mutagenesis to locate and define components of the phasing switch. This project will extend the knowledge of the pathobiology of group A streptococcal infections and could ultimately explain the persistence of streptococci in human populations. Furthermore, this study will yield a wealth of protein sequence information, all proteins which contribute to the pathogenic potential of these bacteria. This information may be useful for the development of a subunit, polyvalent vaccine.